Generally speaking, most RFID transmitting antennas are circularly polarized while most RFID tags are linearly polarized. However, axial ratios of most commercially available mass-produced RFID antennas are normally between 2 dBm and 4 dBm. This is usually not a problem in general use. However, during the test of an RFID tag or tagged product, errors are caused by influences of the axial ratio of the reader antenna when the linearly polarized RFID tag is at different angles. The axial ratio and the gain of the reader antenna also change with frequency.
Two types of RFID test apparatuses exist, which are laboratory-scale instruments (such as instruments from HP™ and Agilent™) and general commercially available readers. The laboratory-scale instruments are too expensive for many end users, so most end users resort to commercially available readers and reader antennas to perform RFID tests. That is to say, in an existing RFID test, an existing RFID transmitting antenna is placed at an elevation angle of 0°, and an object under test rotates being placed within a distance of 1 to 2 meters in front of transmitting reader antenna. However, this test method only scans the horizontal cross section field pattern. Thus, the problem of antenna polarization is not involved during the test.
Strictly, tests of more cross-sections are required to ensure that the tagged product can be read reliably in an actual application. However, axial ratios of most commercially available RFID transmitting antennas are between about 2 dBm and 4 dBm, which are sufficient for most applications. The axial ratios of some special antennas might be between 1 dBm to 2 dBm. That is, when the object under test goes through the rotated scanning and reading, the axial ratio of the transmitting antenna corresponding to the polarization of the RFID tag may influence the measurement accuracy. Therefore, if a set of equipment may effectively correct the errors, the measurement accuracy may be greatly improved.